How forests breathe – Highlights from 30 years of research in the Davos Seehornwald

Just like people, forests breathe. They “inhale” carbon dioxide (CO₂) via photosynthesis to grow and “exhale” CO₂ back into the atmosphere via respiration from plants and soil. The difference between this CO₂ uptake and release is known as the net ecosystem CO₂ exchange (NEE). Tracking NEE is essential as it tells us how much CO₂ is taken up by an ecosystem, assessing its strength as a CO₂ sink. In the Davos Seehornwald, we have been measuring NEE alongside many other environmental variables since 1997. The measurement site, located in a spruce forest close to Davos (Figure 1), is a well-known research platform and part of many international research networks, such as the Integrated Carbon Observation System (ICOS). Data from the station are available open access and visualized online in near-real time. But why is it important to understand the NEE of a forest? Forests, on an annual scale, mostly act as CO₂ sinks, which means that they take up CO₂ from the atmosphere. Hence, they play an important role in the discussion about climate change mitigation. At the same time, rising air temperature and increasingly frequent extreme weather events - like severe droughts and intense storms - can threaten forest functioning, especially in alpine regions. For example, spruce trees are adapted to a cool climate and have shallow roots, so they cannot reach water in deep soil layers during droughts. This furthermore makes them vulnerable to bark beetles, which profit from increased temperature. These complex interactions make it difficult to predict how future climate conditions will affect the forest CO₂ sink strength. The Davos Seehornwald site has one of the longest time series of NEE globally. We investigated NEE over 26 years and found that despite year-to-year variation, the forest took up more CO₂ than was released, indicating that it was a CO₂ sink over this period. The length of the growing season of the forest, i.e. the time during which the forest actively takes up CO₂, did not significantly change over 26 years, even though air temperature increased during this time. This suggests that the spruce trees are adjusting - or acclimating - to changing environmental conditions. However, first signs show that extreme environmental conditions might compromise the CO₂ uptake capacity, e.g. high temperature in summer temporarily limiting the forest CO₂ uptake. The role of the forest as a CO₂ sink under future conditions thus remains uncertain, but long-term and high-quality measurements can help to disentangle the effects of different environmental variables, therefore improving our understanding of how forests function.